The present invention refers to a superposed drilling device wherein impacts are transmitted onto a solid drill bit, and which is provided with an annular drill bit that is impact-coupled to the solid drill bit so that the impact is transmitted from the solid drill bit onto the annular drill bit.
In superposed drilling, used in ground boring and rock drilling, two coaxial drill strings are employed. The inner string is provided with a solid drill bit and the outer string is provided with an annular drill bit surrounding the solid, drill bit. Both strings are advanced rotatingly, the inner string possibly being subjected to impacts. Such impacts may either be generated by an external hammer provided outside the borehole and at the rear end of the inner string, or a deep-hole hammer arranged near the solid drill bit along the longitudinal axis of the inner string. Impacts dealt on the outer string would be absorbed for a large part by the surrounding earth so that they would reach the annular drill bit with a greatly reduced impact energy.
U.S. Pat. No. 3,682,260 describes a superposed drilling wherein a deep-hole hammer strikes the solid drill bit. The solid drill bit and the annular drill bit have cooperating impact transmission surfaces by which the impacts are transmitted from the solid drill bit onto the annular drill bit. In this manner, both drill bits are operated by rotation and impacts, although the annular drill bit has no impact drive means of its own. It is a drawback, however, that upon each blow, the annular drill bit is pulled forward on the outer string so that the outer string is subjected to considerable impact tensile stress. Thus, the threads of the outer pipe string may be damaged or break.
German Patent 21 55 540 describes an improved superposed drilling device wherein the annular drill bit is guided by means of keybeds so as to be longitudinally displaceable at an end piece of the outer string. Also in this case, the impacts are transmitted from the solid drill bit onto the annular drill bit by impact transmitting surfaces so that both drill bits are driven by impacts. The engaging keybeds, provided at the annular drill bit and at the end piece of the outer pipe, which allow for the axial displacement of the annular drill bit upon each impact without stressing the outer pipe string, form bottlenecks in which bore material may gather. In practice, the keybeds are clogged with the loosened bore material so that a displaceability of the annular drill bit is no longer ensured. It may then occur that the impacts are no longer transmitted onto the annular drill bit or that the impacts transmitted onto the annular drill bit do generate considerable tensile stress in the outer pipe string and cause strain thereon. The keybeds or the splines may also jam, thereby impairing the displaceability of the annular drill bit. Drilling devices using an impact transmission from the solid drill bit onto the annular drill bit were not successful in practice due to the above drawbacks.